As an engineer, I know that friction does not depend upon surface area. As a
car nut, I know that wider tires have better traction. How do you explain
this contradiction?
Asked by: Mark Secunda
Answer
This is a good question and one which is commonly asked by students when
friction is discussed. It is true that wider tires commonly have better
traction. The main reason why this is so does not relate to contact patch,
however, but to composition. Soft compound tires are required to be wider in
order for the side-wall to support the weight of the car. softer tires have
a larger coefficient of friction, therefore better traction. A narrow, soft
tire would not be strong enough, nor would it last very long. Wear in a tire
is related to contact patch. Harder compound tires wear much longer, and can
be narrower. They do, however have a lower coefficient of friction,
therefore less traction. Among tires of the same type and composition, here
is no appreciable difference in "traction" with different widths. Wider
tires, assuming all other factors are equal, commonly have stiffer
side-walls and experience less roll. This gives better cornering
performance.
Answered by: Daryl Garner, M.S., Physics teacher MacArthur High School,
Lawton, OK
Friction is proportional to the normal force of the asphalt acting upon the
car tires. This force is simply equal to the weight which is distributed to
each tire when the car is on level ground. Force can be stated as Pressure X
Area. For a wide tire, the area is large but the force per unit area is
small and vice versa. The force of friction is therefore the same whether
the tire is wide or not. However, asphalt is not a uniform surface. Even
with steamrollers to flatten the asphalt, the surface is still somewhat
irregular, especially over the with of a tire. Drag racers can therefore
increase the probability or likelihood of making contact with the road by
using a wider tire. In addition a secondary benefit is that the wider tire
increased the support base and makes it hard to turn the car over in a turn
or in a mishap.
Answered by: Stephen Scholla, B.A., Physics Teacher, Vienna, Virginia